1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, or the like, and more particularly to a developing device for developing a latent image formed on an image bearing member with a liquid type developer having high viscosity.
2. Discussion of the Background
A background developing device used in a background image forming apparatus, which develops a latent image formed on an image bearing member with liquid type developer (hereinafter referred to as developing liquid) having high viscosity, is described in Japanese Laid-open Patent Publication Nos. 07-152254, 07-209922, and 07-219355, for example. In the above-described background image forming apparatus, a photoconductive surface of an image bearing member is uniformly charged by a charge-applying device and is then exposed by an image writing device based on image data. Thereby, a latent image is formed on the image bearing member. A liquid-type developing device then develops the latent image. The developing liquid used in the liquid-type developing device includes a carrier liquid composed of dimethyl siloxane oil, for example, having a viscosity as high as 100 mPa.multidot.s to 10,000 mPa.multidot.s, an insulating character, and toner particles dispersed therein at a high ratio. A container, such as a tank, stores the developing liquid in the liquid-type developing device.
The developing liquid is applied to a surface of a developing roller or a surface of a developing sleeve in the liquid-type developing device to form a thin layer of uniform thickness. The latent image is developed when the developer layer passes through a developing station and the toner therein spreads to the latent image area formed on a photoconductive element, thereby forming a visible toner image thereon. The developing liquid remaining on the surface of the developing roller is removed by a blade and is then collected in the tank. The toner image is then transferred from the photoconductive element to a copy sheet by a transfer device and is then fixed on the copy sheet by a fixing device. The developer remaining on the surface of the photoconductive element after the image transfer is removed by a cleaning device.
In the above-described liquid-type developing device, a developer applying device forms the developer layer on the surface of the developing roller by applying developing liquid thereto by supplying the developing liquid to the developing roller. As the developer applying device, a developer applying roller is generally used, as configured in an ink applying system for a duplicating machine. In order to form a thin layer of uniform thickness on a developer carrier such as a developing roller and a developing sleeve, a sufficient volume of developing liquid needs to be supplied to the developer applying device. As alternative developer applying devices, an applying belt, a hollow cylindroid applying sleeve, or the like, can be employed.
As a method of supplying developing liquid to the developer applying device, such as the developer applying roller, developing liquid may be pumped up by a pump and applied to the developer applying roller. In this method, the pump serves as a developer transfer device and the developing liquid can be supplied to the developer applying roller irrespective of a volume of the developing liquid in the tank.
However, in the case of using the pump, the developing liquid typically does not evenly spread over the surface of the developer applying roller in the axial direction thereof due to its high viscosity. In addition, because a contact area in the circumferential direction of the developer applying roller between the developer applying roller and the developing liquid discharged from an outlet of the pump is typically small, the developing liquid is not sufficiently supplied to the surface of the developer applying roller. In order to supply the developing liquid sufficiently to the developer applying roller, a plurality of outlets of pumps need to be disposed over the surface of the developer applying roller in the axial direction thereof. In this case, a size of the developing device becomes big and its cost increases.
On the other hand, as another method of supplying developing liquid to the developer applying roller, the developer applying roller may be directly immersed in the developing liquid in the tank. In this method, the developing liquid is supplied to the developer applying roller evenly, and a size of the developing device can be small because the mechanism is simple.
However, the following problems arise in the method of immersing the developer applying roller in the developing liquid. When the surface of the developing liquid falls below the lowermost part of the developer applying roller in the tank, the development operation cannot be continued even though some developing liquid remains in the tank, because the developer applying roller does not contact the remaining developing liquid. To ensure that the lowermost part of the developing roller is always immersed in the developing liquid, the storage space of the tank needs to be expanded in the horizontal direction, again resulting in increased size and costs.
When a latent image is developed with the developing liquid of high viscosity, the following problem arises as a result of utilizing the developing liquid of high viscosity. Generally, when developing a latent image, toner in a carrier liquid moves from a developer carrier (e.g., a developing roller) to a latent image on an image bearing member (e.g., a photoconductive element) at a developing station where the developer carrier contacts the image bearing member. When the carrier liquid has high viscosity, the moving speed of toner becomes slower in comparison with a carrier liquid without high viscosity. Accordingly, the developing process needs to proceed slowly. As one solution to solve this problem, a developer carrier having flexibility can be employed to make the nip portion between the developer carrier and the image bearing member wider at the developing station.
Another problem arises as a result of using developing liquid of high viscosity. As illustrated in FIG. 8, when the developing liquid 4 of high viscosity is applied to a developer carrier via a developer applying roller 14, a hanging back phenomenon occurs in which a gap G is produced between a surface of the developing liquid 4 and a surface of the developer applying roller 14 upstream from the bottom part of the developer applying roller 14 in the rotating direction. Specifically, when the developer applying roller 14 rotates in the direction indicated by arrow E in FIG. 8, the developing liquid 4 moves together with the developer applying roller 14 in the same direction as the arrow E. After the developing liquid 4 moves, the gap G is produced because the developing liquid 4 is viscous and its moving speed is slow; that is, the gap G is not filled with the developing liquid 4 quickly. Due to the above-described hanging back phenomenon, the contact time between the developer applying roller 14 and the developing liquid 4 is caused to be short, and as a result the developing liquid 4 cannot be supplied to the developer applying roller 14 sufficiently.